Pneumatic solid fuel distributors are well known in the operation of boilers to improve the distribution of fuel in the boiler. They typically feed bark, “hog fuel” (mixtures of bark and cardboard, sludge, or other waste materials), coal, and/or other solid fuels, separately or combined. They are also referred to as “bark distributors” or “wind swept spouts”. Common designs incorporate a nozzle section (fixed or pivoting) and a pivoting trajectory plate to adjust the angle at which the solid fuel is injected into the boiler. The trajectory plate pivots about a horizontal axis thereby controlling the vertical angle at which the fuel is injected. A more recent design incorporates a flexible trajectory plate and a means for spreading the fuel in a broader horizontal pattern.
Solid fuel boilers are typically constructed as large boxes (up to 100 m2 or more floor area) with heavy steel tubing forming the walls of the box, typically referred to as the front, sides, and rear walls. The tubing is typically 63.5 mm or 76.2 mm outside diameter, arrayed in parallel relationship forming flat panels with the tubes running vertically. The tubes are typically spaced apart about 10-12 mm with a steel membrane or fin bridging the gap. The whole assembly is seal welded together forming an air tight structure. The boiler walls, or tube panels, run vertically to the top of the boiler, up to 30 m or more tall. The walls are fed re-circulating water by headers at their lower extremity. Typically the front wall tubes are bent over more or less horizontally to form the roof of the boiler and the side walls end in relieving headers feeding back to a steam drum. The rear wall either ends in a header or feeds directly into the steam drum. In order to feed fuel and combustion air into the boiler, and for other purposes, the boiler tubes are bent apart to form openings in the tube panel.
The bottom of the boiler may be arranged as a grate type boiler, fluidized bed boiler, or other arrangement. Grate type boilers include traveling grates, vibrating grates, tilting grates, and hydro-grates. Typically the grates cover the bottom of the boiler and are made of heavy cast iron components with slots for combustion air to rise through the grate from a plenum below. The solid fuel lands on the grate and burns there. The ash is dumped off of the grate as the grate moves (rotates like a tank tread), vibrates, or tilts (in sections). Fluidized bed boilers generally have a mass of sand or other media through which a stream of air or boiler flue gas is percolated to fluidize the bed. The fluidized bed acts as a heat sink, turbulent fuel/air mixing system, fuel distribution system, and means for separating fuel and ash in the boiler. Additional combustion air ports, typically called “over fired air” (OFA) are arranged to blow air in above the grate or fluidized bed to help complete the combustion. In all of these arrangements if the fuel is not properly distributed on the grate or fluidized bed, poor combustion can result leading to poor operational efficiencies and high environmental emissions.
In current practice the solid fuel is fed by gravity through large chutes, steeply mounted and about 500 mm square, from a hopper and/or conveyor system above, to the lower portion of the boiler just above the grate or fluidized bed. There are typically multiple chutes penetrating a wall or walls of the boiler. The solid fuel distributor is often integral with and at the bottom of the chute right at the interface with the boiler wall. Mechanical distributors and pneumatic distributors are commonly used. Grate type boilers generally require some type of fuel distribution whereas fluidized bed boilers can be run without them as the fluidized bed can distribute the fuel, albeit inefficiently. In the case of the pneumatic fuel distributor, the solid fuel slides down the chute, passes across the pneumatic orifice and hits the trajectory plate where it is blown off and into the boiler by the pneumatic media. A recent variation attempts to develop a boundary layer of pneumatic media along the top of the trajectory plate such that the solid fuel does not contact the plate. In any case it is the flowing pneumatic media that propels the solid fuel into the boiler.